With the growing demand for ever greater miniaturization of ULSI devices, planarization via CMP becomes an increasingly critical aspect in the fabrication sequence of semiconductor devices. The challenge stems from the multitude and differing nature of materials used in the various layers, the demanding geometries and aspect ratios of the structures, the ever present quest for improved yields via reduction of defects, etc.
Patent applications WO 02/083804 to Costas, US 2002/0177316 A1 to Miller and WO 01/44396 A1 to Sachan, are referenced herewith as indicative of methods and compositions of typical CMPs of the prior art. They reflect the differing natures of CMP compositions, dictated by the tasks/problems they need to address, for example nature of the layers, selectivity, surface roughness and throughput.
CMP slurries can be somewhat simplistically described as consisting of abrasive particulate matter suspended in aq, desirably stable compositions. Such suspensions can contain a host of additives, pH adjusters, leveling agents, oxidants, emulsifiers where needed, and the like. In CMP, the slurry is usually dispensed on a rotating pad in contact with a rotating wafer. Planarization involves a combination of abrasion and chemical reaction at the wafer/slurry interface.
A significant, and generally central component of various metal-polishing slurries, is the oxidizing agent, with hydrogen peroxide and inorganic nitrates appear prevalent. Generally, the oxidizing agent is tailored to suit a given metal to be polished, with copper perhaps being the most challenging. Copper is becoming the metal of choice for interconnect applications, due to its superior electrical conductivity.
While hydrogen peroxide is an attractive oxidizing agent because of reasonable cost, it is not without some serious drawbacks namely poor stability, especially in the presence of transition metals that are known to catalyze decomposition. Another shortcoming of H2O2 is it's less than ideal selectivity. Further, the reaction of peroxides during dissolution of copper, is highly exothermic, making it problematic to maintain temperature stability at the copper/slurry interface, where polishing takes place.
U.S. Pat. No. 6,448,182 to Hall addresses the stability issue through incorporation of stabilizers that are said to reduce, but will not eliminate, decomposition.
The prior art proposes the use of corrosion inhibitors, typically benzotriazoles, as a way to minimize copper oxidation, and improve selectivity. Indeed, benzotriazole is extensively used in the prior art in connection with a host of processes involving copper, taking advantage of its somewhat specific protective, film-forming properties with copper metal, thus serving as a corrosion/oxidation inhibitor for Cu. Its benefit, if any, for metals other than copper, is not indicated.
Some CMP compositions of the prior art are also based on inorganic nitrates. Inorganic nitrates tend to be too aggressive and corrosive, and will not generally favor selectivity, especially for copper
The industry is therefore in constant pursuit of better, task-oriented chemical polishing compositions and methods to satisfy ever-increasing demands for better yields, as geometries continue to shrink.
Mechanistically, CMP is believed to be accomplished by a dual, said to be synergistic, mechanical-chemical process. The mechanical aspect is obtained by applying downward pressure, via a slurry-covered pad, on the surface of the device to be planarized, with the abrasive in the slurry removing unwanted surface material. As such, the mechanism of the abrasive/mechanical action is relatively simple and fairly well understood. On the other hand, the chemical aspect of CMP is more complex, and its interaction with the mechanical aspect of CMP has yet to be fully elucidated. Indeed, it is not quite clear how and to what extent, the oxidizing chemistry of CMP compositions participates in, and brings about, the desired final surface finish, namely smoothness, specularity, freedom from oxides, scratches, and the like. This mechanistic uncertainty especially holds for the planarizing of metals, in particular copper, as partly reflected in above-referenced patents. Indeed, the recent prior art is attempting to address this issue of optimal planarization of copper, in searching for oxidants/chemical etchants that assist and complement polishing/leveling, beyond what is achieve mechanically, albeit concurrently with mechanical abrasion. However, in applying simultaneously/concurrently both chemical and mechanical action of the slurry, the prior art imposes constraints on, and does not fully exploit, chemical polishing, because metal chemical polishing, as opposed to mechanical abrasion, may require different optimal contact times, different process times, etc, depending on the nature and state of the surface to be polished/planarized. Indeed, mechanical abrasion/polishing may, by its very mechanical nature, inherently bring about micro scratches, at the same time as the chemical polishing is trying to “heal” such scratches. Such undertaking is potentially mutually contradictory, if chemical/mechanical polishing is attempted simultaneously.